Two days after the Japanese attack on Pearl Harbor, President Franklin D. Roosevelt made one of his regular radio broadcasts—known as “Fireside Chats”—to the American people. In that broadcast, he set out to prepare the American people for war and to inspire them in their forthcoming struggle.
Roosevelt also set out in detail why the United States was now involved in World War II. In doing so, he gave the impression that America's involvement with the war was somehow inevitable and could not therefore have been avoided.
To see an illustration of this, we need look no further than the second line of FDR's radio address:
Powerful and resourceful gangsters have banded together to make war upon the whole human race.
If these “gangsters”—by which Roosevelt means the Axis Powers of Japan, Italy, and Germany—have indeed declared war upon the human race, then it follows that Americans, as part of the human race, are a part of that conflict. It was always inevitable, then, that the United States would be pulled into conflict sooner or later. The only real question was when they would enter the war, and under what circumstances.
With the benefit of hindsight, Roosevelt is able to look back over a decade of belligerence on the part of the Axis Powers and see that it was inevitable that, at some point, they would represent a clear and present danger to the United States, and the United States would have to respond accordingly.
On this interpretation, the attack on Pearl Harbor was not an isolated event but the inevitable culmination of years of unpunished aggression by Japan and its allies.